Page 195 - WCS - Electrical

WORKSHOP CALCULATION & SCIENCE - CITS

Bakelite

It can be moulded to any shape. It is heat-resistant and highly insulating. It will not absorb oil and moisture. It is
used for bodies of switches, plugs, holders, regulators etc.
Rubber

It has high insulating properties. It is used mainly on lighting cables and for flexible cables. It deteriorates
gradually when exposed to atmosphere. Rubber is being replaced now by elastic plastics such as PVC or
polyethylene which can resist alkalis, acids and mineral oils.

Valcanised India Rubber (VIR)
This is manufactured by treating pure rubber with sulphur. It is stronger than pure rubber and is not affected much
by change in temperatures. It is used as coverings for low and medium voltage wires and cables.

Ebonite or vulcanite
Ebonite or Vulcanite is vulcanised rubber containing about 30% to 50% of sulphur, and subjected to a prolonged
heating at 150°C. The material is hard and can be moulded into different shapes. It is less affected by chemicals
and moisture. It is used for making containers of lead acid batteries, cases for instruments and switchgears,
terminal plates and low voltage panel boards etc. It should not be subjected to heat.

Shellac
It is a good varnish which is used to improve the insulation and moisture resisting properties of paper, cloth, wood,
slate etc.

Enamel
By this, an insulation coating is given on winding wires.
Polychloroprene (PCP)
It is a plastic material used for insulation of cables. It is resistant to oil and petrol. It can be used in conditions of
exposure to sulphur fumes, steam, ammonia, lactic acid and direct sunlight.
Glass
It is heat-resistant and suitable for high temperatures. It is used as insulators, envelopes for lamps, radio tubes
etc.
Quartz
Quartz (Silica) is a good insulator. As it has a very low temperature coefficient of expansion, it does not crack
with sudden variations in temperature. It is used for pyrometer sheaths, for heating elements, sparking plugs, etc.

Porcelain
Porcelain is not so brittle as glass and is very widely used for carrying bare conductors, for making fuse carriers
and other electrical fittings.

Red fibre
Mainly used in motor and transformer winding work, for slot insulation, separators etc.
Insulators classified according to their temperature limits
The permissible temperature limit at which the insulators may be worked safely without deterioration, depends
upon the type and class of the insulation as detailed below. (IS:1271/1958)
Class Y – maximum temperature 90°C
Cotton, silk, paper products, press board, wood, valcanised fibre - not impregnated or immersed in oil.
Class A – maximum temperature 105°C
Cotton, silk, paper products, wood, valcanised fibre when impregnated or immersed in liquid dielectric, varnished
paper and wire enamel (class A).

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CITS : WCS - Electrical - Exercise 18